Influenza vaccines have variable efficacy and protection is often low in the elderly. One hypothesis is that low effectiveness of vaccine in the elderly may be due to a preponderance of influenza-specific but non-neutralizing antibodies. These non-neutralizing antibodies may be against earlier influenza viruses, against denatured or internal viral proteins, or of low avidity. The long-term goal of this research is to develop more effective influenza vaccines by maximizing induction of neutralizing antibodies against the vaccine strain and minimizing the response to denatured glycoproteins, internal proteins, and earlier viruses. Qualitative and quantitative studies of neutralizing and non-neutralizing antibodies in sera of multiply-vaccinated or infected subjects will be undertaken in three Specific Aims. Aim 1: What antibody specificities are present in serum after repeated flu vaccination or infection? Measurement of serum antibodies in vaccinees and infected subjects against the major antigenic drift variants of H3N2 viruses, and in competition assays against monoclonal antibodies specific to those viruses will provide quantitative information on neutralizing versus non-neutralizing, cross-reactive versus strain-specific antibodies, and if the non-neutralizing antibody response is primarily directed against older viruses or against denatured virions. Aim 2: Do the major antigenic regions change in dominance during antigenic drift? The major antigenic sites on the hemagglutinin (HA) have changed in relative dominance over the years. The relative immunodominance of epitopes on the HA and the relative avidity of antibodies will be measured by competition assays, and the results refined by constructing recombinant HA genes engineered to express only one of the major antigenic sites. Aim 3: Devise a vaccine strategy to optimize production of neutralizing antibodies. The results of Aims 1 and 2 will provide a measure of the origins of non-neutralizing antibodies in serum, and allow development of a vaccine strategy to maximize neutralizing antibodies. These experiments are designed to fill a large gap in knowledge of the breadth of human antibody response to influenza vaccines, and to apply this knowledge to vaccine production to improve the ratio of protective, neutralizing antibodies. This may be of particular help in protecting the elderly from influenza.